The invention relates to the continuous casting of metals, and more specifically to the continuous casting, directly from liquid metal, of ferritic-type stainless steel strip, the thickness of which is of the order of a few mm, using the process called xe2x80x9ctwin-roll castingxe2x80x9d.
In recent years considerable progress has been made in the development of processes for casting thin carbon steel or stainless steel strip directly from liquid metal. The process mainly used at the present time is that of casting the said liquid metal between two internally cooled rolls rotating about their horizontal axes in opposite directions and placed parallel to each other, the minimum distance between their surfaces being approximately equal to the thickness that it is desired to give the cast strip (for example a few mm). The casting space containing the liquid steel is defined by the lateral surfaces of the rolls, on which the solidification of the strip starts, and by lateral closure plates made of refractory which are applied against the ends of the rolls. The liquid metal starts to solidify on contact with the outer surfaces of the rolls, on which solidified xe2x80x9cshellsxe2x80x9d form, arrangements being made for these shells to meet in the region of the xe2x80x9cnipxe2x80x9d, that is to say the region where the distance between the rolls is a minimum.
Thin strip made of ferritic stainless steel obtained by twin-roll continuous casting exhibits considerable brittleness, making it difficult for the strip to undergo cold conversion during the usual operations such as decoiling, edge trimming or cold rolling. The poor ductility of twin-roll-cast strip is essentially explained by the very coarse-grained structure resulting from the rapid mode of solidification between the casting rolls, combined with a lengthy residence time at high temperature after the solidified strip has left the bite of the rolls. The high hardness of these ferritic grains supersaturated with interstitial elements, such as carbon and nitrogen, constitutes an aggravating factor with regard to the brittleness of the thin strip.
Several attempts have been made in the past to develop a process for the twin-roll casting of ferritic stainless steels having good ductility. They have relied largely on the addition of known stabilizing elements, such as titanium and niobium, and have imposed compositional limitations on the maximum content of austenite present at high temperature, denoted by the symbol xcex3p. Combined with these compositional conditions were control of the cooling rate, application of hot rolling or control of the temperature at which the cast strip was coiled.
Thus, document EP-A-0,881,305 describes an unstabilized ferritic grade, obtained by direct twin-roll casting of strip, the strip then being coiled at a temperature of less than 600xc2x0 C. The strip is then box-annealed, still in coiled form. Coiling below 600xc2x0 C. makes it possible to limit the precipitation of carbides at the as-cast stage, and thus makes it possible to prevent them coalescing in the form of highly brittle continuous films during box annealing.
Document EP-A-0,638,653 recommends casting a ferritic grade having a chromium content which may be relatively high (13-25%), stabilized with titanium, niobium or aluminium (at least 0.05%), with low carbon and nitrogen contents, and having a negative xcex3p index, xcex3p being the maximum amount of austenite formed at high temperature. This parameter is defined by the Tricot and Castro equation and is calculated using the formula:
xcex3p=420C%+470N%+23Ni%+9Cu%+7Mn%xe2x88x9211.5Cr%xe2x88x9211.5Si%xe2x88x9212Mo%xe2x88x9223V%xe2x88x9247Nb%xe2x88x9249Ti%xe2x88x9252Al%+189.
After casting, a strip is hot rolled with a reduction ratio of greater than 5% in the 950-1150xc2x0 C. range, followed by slow cooling at less than 20xc2x0 C./s or by soaking the strip at high temperature for more than 5 seconds. The strip is then coiled at below 700xc2x0 C. According to that document, the aim is to avoid the formation of austenite at high temperature by imposing a negative xcex3p index in order to prevent the formation of martensite on the strip, which would make it brittle. The presence of stabilizers results, because of the rapid solidification, in fine embrittling precipitates. The hot rolling together with the high-temperature soak and the slow cooling are conducive to precipitation, and especially coalescence, of these precipitates, which thus become innocuous. Cold coiling makes it possible to prevent the formation of brittle intermetallic phases.
Document JP-A-08283845 recommends asynchronous hot rolling of a cast strip with an initial thickness of less than 10 mm, this having the effect of improving the ductility by refining the structure of thin strip by recrystallization. The casting is followed by asynchronous hot rolling and a heat treatment. What is attempted here is to improve the ductility of the thin strip by a recrystallization treatment.
Document JP-A-08295943 uses another estimate of the maximum amount of hot-formed austenite, in the absence of stabilizing elements. This parameter xcex3xe2x80x2p is calculated from:
xcex3xe2x80x2p=420C%+470N%+23Ni%+7Mn%xe2x88x9211.5Cr%xe2x88x9211.5Si%xe2x88x9252Al%+189.
A strip whose xcex3xe2x80x2p index is greater than 25% is cast between rolls, the strip is hot rolled with a reduction ratio of greater than 20% at less than 1200xc2x0 C., then coiled and the coils box-annealed between 700 and 900xc2x0 C. for 4 hours. The aim is to obtain strip with an excellent surface quality, without being especially concerned about its ductility.
All these processes require special heat treatments, possibly necessitating special plants, possibly being expensive in terms of energy and, in the case of box annealing, also lengthy. The economic advantages provided by direct casting of thin strip are therefore to a large part diminished by these processes.
The object of the invention is to provide steelmakers with a process for manufacturing, by twin-roll casting, thin ferritic stainless steel strip that then has to undergo conventional cold conversion steps, without the need for complex or expensive operations such as controlled cooling of the strip or box annealing in order to give said strip good ductility.
With this objective in mind, the subject of the invention is a process for the casting of thin strip having a thickness of less than 10 mm, made of ferritic stainless steel, directly from liquid metal between two rotating cooled rolls having parallel horizontal axes, characterized in that:
the said ferritic stainless steel contains (in percentages by weight) from 11 to 18% chromium, less than 1% manganese, less than 1% silicon and less than 2.5% molybdenum;
the said ferritic stainless steel has carbon and nitrogen contents, the sum of the contents not exceeding 0.05%;
the said ferritic stainless steel contains at least one of the stabilizing elements titanium, niobium, zirconium and aluminium and the sum of their contents is between 0.05 and 1%;
the other elements present are iron and the usual impurities resulting from the smelting;
the xcex3p index of the said ferritic stainless steel is greater than or equal to 30, where:
xcex3p=420C%+470N%+23Ni%+9Cu%+7Mn%xe2x88x9211.5Cr%xe2x88x9211.5Si%xe2x88x9212Mo%xe2x88x9223V%xe2x88x9247Nb%xe2x88x9249Ti%xe2x88x9252Al%+189
and in that, after casting, the thin strip is coiled at a temperature of less than 600xc2x0 C.
The subject of the invention is also thin strip capable of being obtained by the above process.
As will have been understood, the invention consists in combining the presence of one or more stabilizing elements in significant amounts with contents of other alloying elements which nevertheless keep the xcex3p index at a high value, and in coiling the strip at a relatively low temperature. The combination of stabilizing elements and a high xcex3p index, and especially its combination with a low coiling temperature which makes it possible to reconcile these compositional characteristics with very good ductility of the strip without, furthermore, it being necessary to carry out controlled cooling of the strip or a heat treatment which is expensive both in terms of energy and time, is not known in the prior art.
The various characteristics are determined by the following considerations.
A chromium content greater than 11% complies with the usual requirements encountered in ferritic stainless steels. The 18% maximum is justified in that, above this limit, the ductile-brittle transition temperature of stainless steels increases considerably and the invention then becomes inoperable. Chromium also has the tendency to lower the value of the xcex3p index substantially.